The transmission of data over a facility can occur in an asynchronous or synchronous manner. In the asynchronous manner the data bits of a block of data (character, frame, etc.) are transmitted in a strict line sequence, but the blocks of data are not transmitted in a strict time sequence. Since the blocks of data are not transmitted in a strict time sequence, start and stop bits are required to enable the receiving terminal to resynchronize itself to the incoming data. Thus, a disadvantage of using asynchronous data transmission is that it utilizes extra facility capacity for the start and stop bits that need to be transmitted along with a block of data.
In some asynchronous binary data transmission systems the mark to space transition (i.e., a predetermined binary signal transition) is used to signal the start of a data transmission. In such systems the mark state is utilized as one of the binary data states as well as the quiescent state, when no data is being transmitted over the facility. This double usage of the mark state can create a problem when data transmissions are made in a half-duplex manner. When such a system is utilized for half-duplex data transmission (i.e., ping-pong operation) over a 2-wire facility, a control signal (i.e., a carrier signal burst or unique code word) is also required to signal the completion of transmission and the release of the facility by the transmitting party. Undesirably, this control signal adds complexity or utilizes valuable facility capacity.
Thus, there is a continuing need for an asynchronous communication method and circuitry which eliminates the need to utilize start/stop bits or complex control signals to synchronize data transmissions over a facility.